1. Field of Invention
The embodiments described herein relate generally to methods and systems for collecting and processing images in ophthalmology.
2. Background State of the Art
To acquire accurate and repeatable images and measurements of a subject eye, it is desirable to image the subject eye at a fixed distance and at a reproducible location from the imaging device. Therefore, it is important to identify the desirable working distance (also known as the Z distance) between the subject eye and the imaging device. A shift in the position from this fixed distance could lead to inaccurate and less reproducible images, or even errors in measurements.
Corneal topography, for example, is an imaging modality where acquiring accurate and repeatable images is important. Corneal topography uses a method called reflective projection. In this method, a desirable light pattern is projected onto the front surface of a subject eye. A virtual image of the light pattern is then formed near the cornea and captured by an imaging device inside the corneal topographer. The spatial information from this virtual image of the light pattern near the cornea provides information to generate the topographical images of the subject eye. The resulting topographical images are highly sensitive to the distance between the subject eye and the corneal topographer. Since cornea is a high optical power surface, a small variation from a fixed distance between the subject eye and the imaging device can introduce significant measurement errors. If the eye is moved away from the fixed distance at the focal point, the magnification of the virtual image would change and thus result in error of the topographic images or measurements.
Several approaches have been attempted to reduce the error by imaging the subject eye in a fixed position or a more reproducible distance. There are three main approaches, namely 1) reference imaging method, 2) beam triangulation based method and 3) maximum signal method.
FIG. 1 shows an example of the reference imaging method attempting to achieve a reliable fixed distance with a light source 104 and placido object 102 as discussed, for example, in U.S. Pat. No. 5,847,804. The method illustrated in FIG. 1 uses one or more additional imaging device(s) 106 placed off of the optical axis of a main imaging device 108. The distance from the cornea of the subject eye 110 to the main imaging device 108 is determined from the images acquired by the additional imaging device(s) 106. For example, in FIG. 1, additional cameras 106 are placed at a distance perpendicular to the optical axis of the main imaging device 108. An operator then adjusts the distance between the main imaging device 108 and the subject eye 110 until the video image of the eye 110 in additional imaging devices 106 reaches a certain location. This location is identified as the fixed location and images are taken on main imaging device 108 at this location. However, the reference imaging method is highly subjective and difficult to perform for different face sizes and profiles. The desirable fixed location cannot be achieved with high accuracy and repeatability using such method.
FIG. 2 shows an example of the triangulation based method as discussed, for example, in U.S. Pat. No. 6,450,641. Two or more light beams 204 from light sources 202 are used to set a distance of the cornea 208 from the imaging device (not shown). Focusing aid light beams 204 of the imaging device travel at a set angle from the optical axis 206 of the imaging device and are oriented in such a way that the reflected beams 204 either intersect or focus a pattern at a desired distance from the imaging device to achieve the fixed distance. In some examples, an operator of the imaging device is required to adjust the imaging device during an eye exam to locate the intersection of beams 204 reflected from the cornea surface of the subject eye. In another implementation, an operator adjusts the imaging device to position and align a light pattern on the sclera or limbus area of the subject eye as a focusing aid. This triangulation beam method again is subject dependent and presents difficulties for the operator to locate the beam intersection on the cornea. This approach can be uncomfortable for a patient who is subjected to exposure of the multiple aiming beams and is also not user friendly due to the added complexity of additional light beams.
Another method utilized in the attempt to solve the positioning problem is the maximum signal method. In this method, either an aiming light or the main instrument light reflected from the eye is analyzed. In one implementation, the total reflected light reflected from the eye is maximized at a point to achieve best working distance for the subject eye. The distance to achieve maximum reflected light is used as the desired working distance for the main imaging instrument. In another implementation, a series of images are taken during acquisition. Then, the image with the highest signal and/or sharpness is identified and used as the best image for measurement and analysis, without evaluating the proper working distance. These methods and other similar variations are widely practiced in many commercial imaging instruments and cameras. However, these methods are not very reliable because there are other factors affecting the quality and signal of the acquired images such as ambient light.
Thus there is a need for better systems to acquire images of the eye.